1. Field of the Invention
The present invention relates to a resin dispersion for cationic electrodeposition and a low temperature curable cationic electrodeposition coating composition including the same, and more particularly to a low temperature curable and lead-free cationic electrodeposition coating composition in which the electrodeposited coating film thereof has an epoxy-acrylic double-layered structure to give a high functionality, and in which an organic solvent content is minimized.
2. Description of the Related Arts
Since electrodeposition coating has a good efficiency in application and a high corrosion-resistance and can minimize an environmental contamination because of employing water as a solvent, it is widely used for primer coating of cars, household electric appliances, and industrial machines when compared with a brush coating or a spray coating. Recently and widely used epoxy-based cationic electrodeposition coating composition includes cationic epoxy resin onto which amine is added and blocked polyisocyanate. Researches at home and abroad on cationic electrodeposition paint are concentrated on paints related to energy conservation, safety, and environmental considerations. Also, researches on electrodeposition coating, composition are concentrated on developing highly functional and highly efficient electrodeposition paint having various purposes and characteristics.
Generally, a cationic electrodeposition coating composition includes a tin compound such as dibutyl tin oxide in order to improve a cross-linking reaction between a compound containing hydroxyl functional group with a compound containing isocyanate functional group, along with a lead compound as an anticorrosive pigment in order to improve a corrosion-resistance. However, the lead compound becomes a target of restriction on usage in all over the world as a material inducing an environmental contamination. On July 1996 in Japan, an automobile industrial institute established an independent restriction law named as SHREDDER DUST and suggested each automobile manufacturing company to lessen the utilizing amount of lead in the automobiles. In Europe (EU), a law to restrict on crushing, reclamation, incineration of an automobile containing harmful materials such as lead, cadmium, mercury, etc. during the operation of the automobile, will be positively established on the year after 2003.
Therefore, with the movements on restricting the environmental pollutants, researches on reducing or removing the harmful materials such as lead from a coated and dried film of the electrodeposition coating composition are actively progressed.
U.S. Pat. No. 5,908,912 (issued to Kollah et al.) discloses a formation of a lead-free dried film by applying a salt of bismuth compound and a carboxylic acid including amine into a pigment paste as a reaction catalyst for preparing a cationic electrodeposition coating composition. However, there are problems of coagulation of the bismuth salt during storage and a lowering of the pH of the electrodeposition coating composition because of the addition of a large amount of an acid in order to applying the bismuth compound into the cationic electrodeposition coating composition.
Among the researches, studies on reducing an organic solvent emission during coating operations are being developed. The motive for these studies is the regulation of emission of organic solvents for the protection of the environment. As examples of the regulations related to the regulation of emission of organic solvents, the regulation for volatile organic compounds (VOC) of the Environment Office in U.S. and the atmospheric purification law (TA-Luft) in Germany can be noted. The former establishes a guideline for the emission amount of the organic solvents from a coating composition, while the latter restricts the total amount of the organic solvents discharged from the paint consumed for coating one car. Since about 20% of the total amount of carbon dioxide discharged into the world is due to the coating industry (for example, organic solvent and combustion by means of a drying oven), the coating industry is a major cause of the environmental pollution. Hence, the coating industry has become the subject of supervision.
Presently, as an electrodeposition coating composition of primer coating, an epoxy cationic electrodeposition paint is mainly used. However, the epoxy cationic electrodeposition paint is rich in organic solvents and these solvents are evaporated from the electrodeposition tank, thereby emitting an offensive odor from a work-piece. Also, these solvents are emitted during curing and drying, so environmental pollution problems occur. In U.S. Pat. No. 6,147,144, issued to the present Applicants, a cationic electrodeposition coating composition in which the amount of the organic solvent is minimized, is disclosed.
Further, even though the conventional epoxy cationic electrodeposition paint has a good corrosion-resistance and a good adherence, this is lacking in properties such as a weather-resistance, a yellowing resistance, etc.
It is an object of the present invention, considering problems, to provide a low-temperature curable resin dispersion of a cationic electrodeposition, of which dried film is free from lead, and which includes a minimized amount of organic solvents and has an epoxy-acrylic double-layered structure to improve a weather-resistance and a yellow resistance, and a method of preparing therof.
Another object of the present invention is providing a cationic electrodeposition coating composition including the above-described resin dispersion of the cationic electrodeposition and a pigment paste composition, and a method of preparing thereof.
To accomplish the above object, there is provided in the present invention a resin dispersion of a cationic electrodeposition including an aqueous dispersion. The aqueous dispersion is prepared by mixing about 35-45 parts by weight of a cationic electrodeposition resin, about 55-65 parts by weight of deionized water, about 0.3-1.5 parts by weight of an acid for neutralization, about 1-5 parts by weight of a reaction product of manganese phosphate and an acid diluted in an deionized water to 10% and about 0.5-1 parts by weight of a cationic surfactant.
The cationic electrodeposition resin is prepared in the presence of an organic solvent from (a) about 40-60% by weight of a cationic electrodeposition synthetic resin produced by an epoxy-amino addition reaction (b) about 5-10% by weight of an acrylic cationic electrodeposition resin having an amino group (c) about 1-3% by weight of a fatty acid ester resin synthesized by an esterification reaction of styrene-allylalcohol copolymer and fatty acid and (d) about 30-50% by weight of polyurethane cross-linking agent produced from a mixture of blocked polymethylene polyphenyl polyisocyanate and blocked isophorone diisocyanate.
The resin dispersion of the cationic electrodeposition is prepared by the following method including the steps of preparing a cationic electrodeposition resin, preparing an Is aqueous dispersion, removing organic solvents having a low boiling point from the aqueous dispersion by extracting and then filtering.
The cationic electrodeposition resin is prepared in the presence of an organic solvent from (a) about 40-60% by weight of a cationic electrodeposition synthetic resin produced by an epoxy-amino addition reaction (b) about 5-10% by weight of an acrylic cationic electrodeposition resin having an amino group (c) about 1-3% by weight of a fatty acid ester resin synthesized by an esterification reaction of styrene-allylalcohol copolymer and fatty acid and (d) about 30-50% by weight of polyurethane cross-linking agent produced from a mixture of blocked polymethylene polyphenyl polyisocyanate and blocked isophorone diisocyanate.
The aqueous dispersion is prepared by mixing about 35-45 parts by weight of the cationic electrodeposition resin, about 55-65 parts by weight of deionized water, about 0.3-1.5 parts by weight of an acid for neutralization, about 1-5 parts by weight of a reaction product of manganese phosphate and an acid diluted in an deionized water to 10% and about 0.5-1 parts by weight of a cationic surfactant.
Preferably, a solid content in the resin dispersion of the cationic electrodeposition is about 30-40% by weight and a mean size of particles contained in this dispersion is about 60 nm or less. And a preferred amount of organic solvents in this dispersion is about 0.5% by weight or less.
The other object of the present invention is accomplished by a cationic electrodeposition coating composition comprising an aqueous dispersion and a pigment paste composition. The aqueous dispersion is prepared by mixing about 35-45 parts by weight of a cationic electrodeposition resin, about 55-65 parts by weight of deionized water, about 0.3-1.5 parts by weight of an acid for neutralization, about 1-5 parts by weight of a reaction product of manganese phosphate and an acid diluted in an deionized water to 1 0% and about 0.5-1 parts by weight of a cationic surfactant.
The cationic electrodeposition resin is prepared in the presence of an organic solvent from (a) about 40-60% by weight of a cationic electrodeposition synthetic resin produced by an epoxy-amino addition reaction (b) about 5-10% by weight of an acrylic cationic electrodeposition resin having an amino group (c) about 1-3% by weight of a fatty acid ester resin synthesized by an esterification reaction of styrene-allylalcohol copolymer and fatty acid and (d) about 30-50% by weight of polyurethane cross-linking agent produced from a mixture of blocked polymethylene polyphenyl polyisocyanate and blocked isophorone diisocyanate.
A method of preparing the cationic electrodeposition coating composition also is provided in the present invention. The cationic electrodeposition coating composition is prepared by preparing a cationic electrodeposition resin, preparing an aqueous dispersion, preparing a resin dispersion of a cationic electrodeposition by removing organic solvents having a low boiling point from the aqueous dispersion by extracting and then filtering, and then mixing thus obtained cationic electrodeposition resin dispersion with a pigment paste composition.
The cationic electrodeposition resin is prepared in the presence of an organic solvent from (a) about 40-60% by weight of a cationic electrodeposition synthetic resin produced by an epoxy-amino addition reaction (b) about 5-10% by weight of an acrylic cationic electrodeposition resin having an amino group (c) about 1-3% by weight of a fatty acid ester resin synthesized by an esterification reaction of styrene-allylalcohol copolymer and fatty acid and (d) about 30-50% by weight of polyurethane cross-linking agent produced from a mixture of polymethylene polyphenyl polyisocyanate and blocked isophorone diisocyanate.
The aqueous dispersion is prepared by mixing about 35-45 parts by weight of the cationic electrodeposition resin, about 55-65 parts by weight of deionized water, about 0.3-1.5 parts by weight of an acid for neutralization, about 1-5 parts by weight of a reaction product of manganese phosphate and an acid diluted in an deionized water to 10% and about 0.5-1 parts by weight of a cationic surfactant.
The preferably employed pigment paste composition includes (a) about 15-30% by weight of pigment grinding vehicle (b) about 2-6% by weight of an anticorrosive pigment and (c) about 0.7-2.3% by weight of dibutyl tin oxide. The ratio of solid content of the pigment/pigment grinding vehicle is in a range of 1/0.2-1/0.45.
Particularly, the pigment grinding vehicle is obtainable from about 25-35% by weight of polyglycidyl ether of bisphenol A, about 5-12% by weight of propyleneglycol monomethyl ether acetate, about 8-15% by weight of partially blocked isocyanate cross-linking agent, about 25-40% by weight of ethylene glycol monobutyl ether, about 10-20% by weight of organic tertiary amino acid salt and about 0-5% by weight of deionized water. Further, the epoxy equivalent weight of the polyglycidyl ether of bisphenol A is in a range of about 900-1200.
In addition, the preferred anticorrosive pigment is at least one selected from the group consisting of bismuth hydroxide, bismuth trioxide, bismuth oxide, aluminum tri-polyphosphate hydrate and magnesium aluminum hydroxide carbonate hydrate.
The cationic electrodeposition resin applicable in the present invention is a major component for forming a film in the electrodeposition coating composition. Some of these known cationic electrodeposition resin are illustrated in U.S. Pat. Nos. 3,663,839; 3,984,299; 3,947,338; 3,947,339; and 6,130,274. These film forming resins represent a resin having an amino group formed by an addition reaction of polyepoxide with a primary, secondary, or tertiary amine. In the present invention, as for the cationic electrodeposition synthetic resin formed by an addition reaction of polyepoxide and amine, a resin having the following structural formula can be illustrated: 
In the above-illustrated structure, R., R2 and B are as follows and n is an integer of 3or 4.
If the amount of the cationic electrodeposition synthetic resin having an amino group used is less than 40% by weight, mechanical properties of the dried coating layer are weak, and if the amount of the cationic electrodeposition synthetic resin used exceeds 60% by weight, the manufacture of an aqueous dispersion is difficult. Therefore, the amount of the cationic electrodeposition resin having an amino group used is preferably in the range of about 40-60% by weight, and more preferably about 45-55% by weight.
In addition, as another cationic electrodeposition resin, an acrylic cationic electrodeposition resin having an amino group which is copolymerized with butyl acrylate, methyl acrylate, hydroxy ethyl acrylate, styrene, methyl methacrylate, (N,N-dimethyl)aminoethyl metacrylate, etc., can be illustrated.
If the amount of the acrylic cationic electrodeposition resin used is less than 5% a by weight, a dried coating film having an epoxy-acrylic double-layered structure cannot be obtained, and if the amount of the acrylic cationic electrodeposition resin used exceeds 10% by weight, the epoxy resin and acrylic resin are not compatible, thereby affecting the surface appearance and gloss. Therefore, the amount of the acrylic cationic electrodeposition resin used is preferably in the range of about 5-10% by weight, and more preferably about 7-9% by weight.
Usually these resins having an amino group are used together with a blocked polyisocyanate curing agent. Isocyanate is so completely blocked that it can be mixed with the resin having an amino group. Otherwise, isocyanate is partially blocked in such a manner that it can react with the backbone of a resin. As for the blocking agent of the polyisocyanate compound, at least one compound selected from the group consisting of methyl ethyl ketoxime, dimethyl pyrazole, diethyl malonate, ethylene glycol monobutyl ether and diethylene glycol monobutyl ether can be preferably utilized. As for the blocked polyisocyanate curing agent which can be dissociated at a low temperature, the compounds having the following structural formulae can be illustrated: 
If the amount of the blocked polyisocyanate curing agent used is less than 30% by weight, properties such as pencil hardness, corrosion-resistance, etc., are diminished. If the amount of the blocked polyisocyanate curing agent used exceeds 50% by weight, the preparation of an aqueous dispersion is difficult and properties such as impact-resistance, flexibility, etc., are diminished. Therefore, the amount of the blocked polyisocyanate curing agent used is preferably in the range of about 30-50% by weight, and more preferably 35-45% by weight.
Particularly, the composition of the present invention comprises a fatty acid ester synthetic resin synthesized by an ester reaction of styrene-allylalcohol and fatty acid, in order to improve a smoothness of the electrodeposited coating film and to inhibit a pinhole and cratering characteristic. As for the fatty acid ester synthetic resin, a resin having the following structural formula can be illustrated: 
The molecular weight (weight-average molecular weight) of these fatty acid ester resins is preferably in the range between about 2000-40,000, and more preferably is between about 3000-30,000. If the amount of these fatty acid ester resins used is less than 1% by weight, an improvement of the smoothness of the electrodeposited coating film and the restraint of the pinhole and cratering characteristic cannot be achieved. However, if the amount of these fatty acid ester resins used exceeds 3% by weight, mechanical properties of the dried coating film such as pencil hardness is deteriorated. Therefore, the amount of the fatty acid ester resin used is preferably in the range of about 1-3% by weight, and more preferably about 1.5-2.5% by weight.
The cationic electrodeposition composition according to the present invention may contain pigments, solvents and additives such as a surface improving additive, a surfactant, a thickening agent, a curing catalyst, etc. As the curing catalyst, a metallic salt of organic acids such as cobalt acetate, chrome octoate, chrome naphthenate, lead acetate and dibutyl tin oxide may be usually used.
The method of preparing the cationic electrodeposition coating composition according to the present invention will be described in detail below.
First, a cationic electrodeposition resin is prepared in the presence of an organic solvent having a low boiling point of about 140xc2x0 C. or less from (a) about 40-60% by weight of a cationic electrodeposition synthetic resin produced by an epoxy-amino addition reaction (b) about 5-10% by weight of an acrylic cationic electrodeposition resin having an amino group (c) about 1-3% by weight of a fatty acid ester resin synthesized by an esterification reaction of styrene-allylalcohol copolymer and fatty acid and (d) about 30-50% by weight of polyurethane cross-linking agent produced from a mixture of blocked polymethylene polyphenyl polyisocyanate and blocked isophorone diisocyanate.
As the organic solvents having the low boiling point, methyl isobutyl ketone, xylene, toluene, methyl ethyl ketone, etc. can be illustrated.
The cationic electrodeposition synthetic resin obtained by an epoxy-amino addition reaction may be manufactured by using an epoxy resin, polycaprolactonediol, bisphenol A, and benzyl dimethyl amine.
Thereafter, about 35-45% by weight of thus prepared cationic electrodeposition resin, about 55-65 parts by weight of deionized water, about 0.3-1.5 parts by weight of an acid for neutralization, about 1-5 parts by weight of a reaction product of manganese phosphate and an acid diluted in an deionized water to 10%, and about 0.5-1 parts by weight of a cationic surfactant, are mixed to prepare an aqueous dispersion. The aqueous dispersion is extracted under reduced pressure to remove the organic solvents having the low boiling point. In the following, the aqueous dispersion is filtered to prepare the cationic electrodeposition coating composition.
During the process of using the acid for neutralization, the degree of neutralization of the existing amino group is preferably in the range of about 20-100%, and more preferably is about 40-70%. According to a suitable neutralization method, an acid for neutralization such as formic acid, acetic acid, lactic acid and phosphoric acid, a surfactant, deionized water, etc., are added and dispersed into water by a high-speed stirring so that an excellent resin of an aqueous dispersion may be obtained.
Successively, a solvent stripping of the organic solvent having the low boiling point is performed at about 40-80xc2x0 C. and at about 30-90 mmHg, and more preferably is performed under a reduced pressure of about 50-70 mmHg. Then, organic solvents having a high boiling point over 140xc2x0 C., which has been included in the preparation of the cationic electrodeposition synthetic resin obtained by an epoxy-amino addition reaction, the acrylic cationic electrodeposition resin having an amino group, and the fatty acid ester resin synthesized by an esterification reaction of styrene-allylalcohol copolymer and fatty acid, remains. Meanwhile, organic solvents having the low boiling point such as methyl isobutyl ketone, xylene, toluene, methyl ethyl ketone, etc., are removed.
After implementing the solvent stripping process for making the solid content of the aqueous dispersion of the cationic electrodeposition resin being about 20-60% by weight, and more preferably about 30-40% by weight, the aqueous dispersion is filtered through diatomaceous earth in such a manner that an improved cationic electrodeposition coating composition can be obtained. The average particle size of the improved cationic electrodeposition coating composition is less than 60 nm and the organic solvent content is minimized.
The method of preparing the first component of the cationic electrodeposition coating composition, the cationic electrodeposition resin dispersion has been described above. Hereinbelow, a method of preparing the second component, a pigment paste composition will be described.
First, a pigment grinding vehicle for the cationic electrodeposition coating composition is prepared from about 25-35% by weight of polyglycidyl ether of bisphenol A, about 5-12% by weight of propyleneglycol monomethyl ether acetate, about 8-15% by weight of partially blocked isocyanate cross-linking agent, about 25-40% by weight of ethylene glycol monobutyl ether, about 10-20% by weight of organic tertiary amino acid salt and about 0-5% by weight of deionized water.
If the amount of the polyglycidyl ether of bisphenol A used is less than about 25% by weight, properties such as corrosion-resistance, water resistance, etc. are deteriorated, and if the amount of the polyglycidyl ether of bisphenol A used exceeds 35% by weight, mechanical properties of the dried coating layer such as hardness is diminished. Therefore, the amount of the polyglycidyl ether of bisphenol A used is preferably in the range of about 25-35% by weight, and more preferably about 27-32% by weight.
If the amount of the propyleneglycol monomethyl ether acetate used is less than 5% by weight, the process of melting the polyglycidyl ether resin of bisphenol A is poorly performed. If the amount of the propyleneglycol monomethyl ether acetate used exceeds 12 percent by weight, the stability of the paint manufactured with the pigment grinding vehicle is deteriorated. Therefore, the amount of the propyleneglycol monomethyl ether acetate used is preferably about 5-12% by weight, and more preferably about 7-1 0% by weight.
If the amount of the partially blocked isocyanate cross-linking agent used is less than 8% by weight, the hardness of the coating layer is lessened. If the amount of the partially blocked isocyanate cross-linking agent used exceeds 15% by weight, the mechanical property of the coating layer is lowered due to the increase of the hardness of the coating layer. Therefore, the amount of the partially blocked isocyanate cross-linking agent used is preferably in the range of about 8-15% by weight, and more preferably about 10-13% by weight.
If the amount of ethylene glycol monobutyl ether used is less than 25% by weight, the stability of the electrodeposition paint is poor, and if the amount of the ethylene glycol monobutyl ether used exceeds 40% by weight, the organic solvent content is increased. So, the amount of the ethylene glycol monobutyl ether used is preferably in the range of about 25-40% by weight, and more preferably about 30-35% by weight.
If the amount of the organic tertiary amino acid salt used is less than 10% by weight, the water-miscibility is insufficient. And if the amount of the organic tertiary amino acid salt used exceeds 20% by weight, the property of water resistance is poor. So, the amount of the organic tertiary amino acid salt used is preferably in the range of about 10-20% by weight, and more preferably about 13-16% by weight.
At this time, if the polyglycidyl ether of bisphenol A has an epoxy equivalent weight less than about 900, chemical properties of the coating layer such as water resistance, corrosion-resistance, etc., are poor. Therefore, the epoxy equivalent weight of the polyglycidyl ether of bisphenol A used is preferably about 900-1200 (or 1400-2800 by weight-average molecular weight).
The pigment paste composition includes the pigment grinding vehicle prepared by the above-described method, pigment ingredients such as carbon black or titanium dioxide, a catalyst such as dibutyl tin oxide and an anticorrosive pigment for improving the corrosion-resistance.
In the pigment paste composition, the amount of the pigment grinding vehicle used is in the range of about 15-30% by weight and preferably, about 17-25% by weight. If the amount of the pigment grinding vehicle is less than 15% by weight, a viscosity increases during storing, thereby weakening a storage stability. If the amount of the pigment vehicle exceeds 30% by weight, the storage stability is increased. However, some properties such as water-resistance, chemical-resistance, etc. of a dried film of the cationic electrodeposition coating composition are deteriorated.
In order to improve the corrosion-resistance of the dried film, an anticorrosive pigment such as a bismuth compound containing bismuth hydroxide, bismuth trioxide, bismuth oxide, etc., aluminum tri-phosphate hydrate, magnesium aluminum hydroxide carbonate hydrate, etc. can be included in the pigment paste composition. The amount of the anticorrosive pigment is in the range of about 2-6% by weight, and more preferably about 3-5% by weight. If the amount of the anticorrosive pigment is less than 2% by weight, the corrosion-resistance of the dried film of the cationic electrodeposition coating composition is deteriorated and if the amount exceeds 6% by weight, the viscosity of the pigment increases to generate a coagulating phenomenon during storing.
Further, a tin compound such as dibutyl tin oxide can be included in the pigment paste composition as a curing catalyst. The amount used of the tin compound is in the range of about 0.7-2.3% by weight, and more preferably about 1-2% by weight. If the amount of the tin compound is less than 0.7% by weight, a curing characteristic is weakened and so mechanical properties and chemical properties are weakened after drying the cationic electrodeposition coating composition. If the amount of the tin compound exceeds 2.3% by weight, the crosslinking density increases after the drying and the chemical properties are improved, however, the mechanical properties such as impact resistance and flexibility are deteriorated.
In addition, the ratio of the solid content of the pigment/pigment grinding vehicle in the pigment paste composition is in the range of about 1/0.20-1/0.45, and more preferably in the range of about 1/0.25-1/0.35. If the ratio of the solid content of the pigment/pigment grinding vehicle of the pigment paste composition is less than 1/0.20, the mechanical properties and the chemical properties of the dried film of the cationic electrodeposition coating composition are deteriorated, and if exceeds 1/0.45, the stability of the pigment paste composition is deteriorated to generate a coagulating phenomenon.